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Abstract

Background

The physical health status of vegetarians has been extensively reported, but there
is limited research regarding the mental health status of vegetarians, particularly
with regard to mood. Vegetarian diets exclude fish, the major dietary source of eicosapentaenoic
acid (EPA) and docosahexaenoic acid (DHA), critical regulators of brain cell structure
and function. Omnivorous diets low in EPA and DHA are linked to impaired mood states
in observational and experimental studies.

Methods

We examined associations between mood state and polyunsaturated fatty acid intake
as a result of adherence to a vegetarian or omnivorous diet in a cross-sectional study
of 138 healthy Seventh Day Adventist men and women residing in the Southwest. Participants
completed a quantitative food frequency questionnaire, Depression Anxiety Stress Scale
(DASS), and Profile of Mood States (POMS) questionnaires.

Conclusions

Background

Although adherence to vegetarian diets has been associated with physical health benefits,
most notably a low risk of mortality from ischemic heart disease [1], vegetarian mental health is not well documented. Emerging evidence suggests that
fish consumption has a protective effect on mental health due to the long-chain omega-3
fatty acid content [2]. Traditional vegetarian diets omit all flesh foods, and low intakes of the long-chain
omega-3 fats, eicosapentaenoic acid [EPA] and docosahexaenoic acid [DHA], have been
widely reported in vegetarians [3-6]. EPA and DHA favorably impact neural function by displacing the long-chain omega-6
fatty acids in brain cell membranes, particularly arachidonic acid [AA] [7]. AA is a key substrate for the synthesis of proinflammatory eicosanoids and downstream
cytokines [8], which can adversely impact mental health via a cascade of neuroinflammation [9,10].

Vegetarians must rely on limited endogenous production of EPA and DHA from the short-chain
omega-3 fatty acid, α-linolenic acid [ALA] in plant foods [11,12]. Yet, high intake of the plant-derived omega-6 fatty acid, linoleic acid [LA], characteristic
of vegetarian diets [3-5], actually reduces omega-3 tissue incorporation [13-15]. A recent study reported the average omega-6 to omega-3 ratio in red blood cell phospholipids
of vegans was 18.6 compared to 9.9 in omnivores [3], suggesting a higher overall inflammatory milieu for vegetarians. Research is needed
regarding whether low omega-3 intake in vegetarians affects their mental health. The
objective of this cross-sectional study was to compare the mood of vegetarians who
never eat fish with the mood of their healthy omnivorous counterparts.

Methods

Volunteers from Seventh Day Adventist (SDA) communities in the Phoenix, Arizona and
Santa Barbara, California metropolitan areas in the United States were recruited for
this study since they represent a particularly homogeneous group in terms of lifestyle
characteristics [16]. Approximately one-third of Adventists eat no meat, fish, or poultry [17]. Individuals were excluded if pregnant or lactating, diagnosed with chronic disease
affecting mental state, or regular users of medications or supplements known to influence
mood. This trial was approved by the Institutional Review Board at Arizona State University.
Sixty-four vegetarians and seventy-nine non-vegetarians read a letter of consent and
completed an anonymous survey which took approximately 30 minutes. Completion of the
survey indicated consent. The survey included three parts: a general health history
questionnaire, a food frequency questionnaire (FFQ), and two psychometric tests, the
Depression Anxiety Stress Scale (DASS) and the Profile of Mood States (POMS).

The healthy history questionnaire captured demographic information including a categorical
question about education level completed, supplement and medication intake, and a
validated total weekly leisure activity screening tool [18], since it is well established that physical activity level can modulate mood [19]. Participants were asked to report their weekly frequency of strenuous, moderate,
and light intensity activities which were multiplied by nine, five, and three METs,
respectively, and total weekly leisure activity was calculated by summing the products
of the separate components. Multiple level education data was collapsed into two categories
(college and no college) for analysis.

The FFQ was a 152-item quantitative questionnaire which was previously validated for
use in estimating the intake of n-3 fatty acids in cardiac patients [20]. We modified the FFQ slightly with the addition of other foods commonly consumed
by vegetarians, and in addition to estimating n-3 fatty acid intake, we estimated
the intakes of other major fatty acids using the USDA's Food and Nutrient Database
for Dietary Studies (United States Department of Agriculture, 2008). Participants
indicated their portion sizes and intake frequency for each food item. The estimated
intake of each type of fatty acid was calculated by multiplying intakes by the selected
serving size: small (0.5), medium or standard USDA serving size (1.0), and large (1.5),
and by the frequency of consumption selected: once a month, less than once a week,
1-2 times a week, 3-4 times a week, 5-6 times a week, daily, and more than once a
day, and entered into the spread sheet as 1, 3, 6, 14, 22, 30, or 60, respectively.
Estimated intake of each fatty acid type was calculated using Microsoft Excel 2007
software (Microsoft Corp, Seattle, WA).

The DASS was designed to measure three related but distinct negative affective states
in nonclinical populations: depression (D) which assesses dysphoria, hopelessness,
devaluation of life, self-deprecation, lack of interest, anhedonia, and inertia; anxiety
(A) which assesses autonomic arousal, skeletal musculature effects, situational anxiety,
and subjective experience of anxious affect; and stress (S) which assesses difficulty
relaxing, nervous arousal, and being easily agitated, irritable, overreactive, and
impatient [21]. This scale has been validated for use in nonclinical populations and in research
settings [22,23]. The 42-item questionnaire contains 14 complete sentence items for each of the three
areas, and subjects are asked to use 4-pt severity/frequency scales to rate the extent
to which they have experienced each state over the past week, with higher scores indicating
a greater degree of mood disruption. The DASS takes approximately 10 minutes to complete.
The total score is determined by summing the three subscale scores (reported normative
score for nonclinical population: 18.38) [24]. Reliability of the three scales is considered excellent, with Cronbach's alpha at
.95 for D, .90 for A, .93 for S, and .97 for total score [23]. Test-retest reliability is also excellent with .72 for D, .79 for A, and .81 for
S [22]. The DASS-D correlates with the Beck Depression Inventory (BDI-II; r = .74), the
standard clinical measure of depression [21]. The DASS has adequate convergent and discriminant validity (CFI = .93) [23].

The POMS (Educational and Industrial Testing Service, San Diego, CA) estimates the
intensity of mood disturbance, is easy to administer, and is one of the most widely
used and accepted mood scales in healthy populations [25,26]. It consists of 65 adjectives rated on a 5-pt Likert-type scale ranging from 'not
at all' to 'extremely', and covers six mood domains: tension-anxiety (POMS-T); depression-dejection
(POMS-D); anger-hostility (POMS-A); vigor-activity (POMS-V); fatigue-inertia (POMS-F);
confusion-bewilderment (POMS-C); the total POMS score was computed by summing the
five negative domain scores (T, D, A, F, and C) and subtracting the vigor (V) score.
Higher scores indicate a greater degree of mood disturbance (normative mean scores:
males 14.8, females 20.3) [26]. These data are based on 'during the past week, including today' time frame. The
POMS has a high degree of reliability, with reported Cronbach's alpha ranging from
.84 to .95 [25]. Total POMS is moderately-to-highly correlated with standard scales (the Visual Analog
Mood Scales, the State-Trait Anxiety Inventory, and the Beck Depression Inventory)
with coefficients ranging from .72 to .79 [26].

Since vegetarians have a relatively higher prevalence of anemia due to lower intake
of iron and vitamin B12 [27] and because this condition can adversely affect mood [28], the first half of our participants were tested for anemia (n = 63). A finger stick
was administered by a trained phlebotomist, and hematocrit was determined from a drop
of capillary blood.

Descriptive statistics were reported for all outcome measures and data are reported
as mean ± SE. Independent sample t-tests and Chi-square tests were utilized to examine
the impact of differences between group characteristics. Mood scores were normalized
by square root transformation prior to analyses, and differences between groups were
examined using independent sample t-tests and univariate ANCOVA when adjusting for
confounding variables. Dietary data could not be normalized; the Mann-Whitney U test
was utilized to examine differences between groups. Spearman's correlation was used
to assess relationships between variables. Data were analyzed using The Statistical
Package for the Social Sciences (SPSS, versions 15 and 16 for Windows, 2006, Chicago
IL) and p values < 0.05 were considered significant.

Results

Participant characteristics

Data are reported for 138 participants, 107 participants were from the Phoenix SDA
community and 31 were from the Santa Barbara SDA community. Mean population characteristics
including mood scores did not differ by location. Five participants (one non-vegetarian
and four vegetarians) of our initial survey population (n = 143) were removed from
the data set prior to analyses due to either reported anti-depressant use (an exclusion
criteria) or extreme reported total POMS score (> 3SD from the mean) leaving a total
of 138 participants; and for the diet analysis only, three participants (all VEG)
were excluded due to extreme dietary polyunsaturated fatty acid intakes. Mean age,
BMI, education, and PA values for these eight excluded participants did not vary significantly
from the population means. Participants were grouped as omnivores (OMN: n = 78) or
vegetarians (VEG: n = 60). VEG were defined as participants who excluded all flesh
foods. Of the OMN participants, 82% were at least monthly fish-eaters, and about one-third
ate fish weekly.

These results challenge what is known about the link between dietary fats and brain
function and suggest an unrecognized benefit of vegetarian diets which are naturally
low in the long-chain omega 3 fats. Several randomized, placebo-controlled trials
have demonstrated beneficial effects of fish or fish oil on mood states as indicated
by significant reductions in POMS scores in healthy volunteers [32,33]. In these subject populations, markers of oxidative stress were reduced in the fish-supplemented
groups concomitant with improved mood states, findings that support earlier descriptive
data directly linking oxidative stress and psychological distress [34]. Vegetarians are generally characterized by high circulating concentrations of antioxidants
and reduced oxidative stress [35,36], attributed to high intakes of plant foods in this population. Markers of oxidative
stress were not collected in the present trial, and mechanisms to explain associations
between vegetarian diets and improved mood states have not been explored.

The vegetarians reported significantly higher LA and ALA intakes than the omnivores,
a profile well established in the literature [37]. High intakes of these polyunsaturated fatty acids have been shown to inhibit the
activity of desaturases required for endogenous formation of longer chain metabolites;
thus, as blood levels of LA rise, levels of AA reportedly do not, and inflammation
is decreased [38-40]. Moreover, absolute amounts of LA and ALA have been shown to be more important than
relative proportions of these fatty acids when it comes to conversion efficiency [41,42]. Based on his latest review of published data on dietary intake of polyunsaturated
fatty acids by vegetarians, Sanders [43] concluded that plasma proportions of EPA and DHA may be adequate in vegetarians as
long as there is high ALA intake which lowers the LA/ALA ratio. A number of studies
that compared dietary fat intake and blood lipids of omnivores and vegetarians found
that AA content of serum lipids was either similar or significantly lower in vegetarians
than in the omnivores despite higher LA concentrations and lower EPA and DHA concentrations
[3,4,6,11,44,45]. Thus, perhaps low intakes of EPA and DHA are not linked to adverse mood state in
vegetarian populations, because intakes of ALA are generally high, AA intakes are
low, and conversion of LA to AA is regulated.

A major limitation of our study was not measuring blood fatty acid concentrations
or inflammatory markers. However, the use of food frequency questionnaires is considered
effective in capturing omega-3 fatty acid intake since sources are consumed relatively
infrequently and can be reported fairly accurately [46]. Our data may have been influenced by response bias, since SDA vegetarians may be
more defensive about their diet choice than SDA omnivores, however, participants were
not aware that the focus of the study was on vegetarian diets. Also, vegetarians may
make better dietary choices, and may generally be healthier and happier [17]. In exclusively surveying the SDA community, we were able to identify vegetarian
participants and analyze a relatively homogenous population of vegetarians and omnivores,
thus minimizing potentially confounding lifestyle differences. These results, however,
may not be generalizable to non-SDA populations.

Conclusions

While dietary intake of EPA and DHA has an important role in brain function, we found
no evidence that the absence of direct intake of these fatty acids in vegetarians
adversely affects mood state. Features of the vegetarian diet profile such as higher
intake of total polyunsaturated fat and negligible arachidonic acid intake may help
explain the favorable mood profile we observed with vegetarian diets. Future research
exploring the effect of dietary fat modifications on omnivore mental health may have
public health importance.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

BLB participated in the study design and acquisition of data, performed the statistical
analysis and interpretation of results, and drafted the manuscript. CSJ participated
in the study design and acquisition of data, assisted in the statistical analysis
and interpretation of results, and edited the manuscript. DRD participated in the
dietary data analysis. All authors read and approved the final manuscript.

Acknowledgements

We would like to thank Dr. John Westerdahl, Director of the Bragg Health Foundation
in Santa Barbara, California, for his assistance in enlisting participants in the
Santa Barbara SDA community.

Goyens PL, Spilker ME, Zock PL, Katan MB, Mensink RP: Conversion of alpha-linolenic acid in humans is influenced by the absolute amounts
of alpha-linolenic acid and linoleic acid in the diet and not by their ratio.